1. Field of the Invention
This invention relates to a signal processing device, and more particularly, to compression processing of image signals.
2. Description of the Related Art
Recently, so-called digital VCRs (video cassette recorders) have been considered in which digital signal processing of image signals of so-called VCR's is performed using magnetic tape having a width of ½ inches or 8 millimeters for recording moving-image signals.
The digital VCR performs various kinds of image processing using correlation between image in time. One example of such processing is frame correlation image compression processing which uses correlation between images in time.
FIG. 5 is a diagram showing the principle of the frame correlation image compression processing.
In FIG. 5, there are shown a picture frame n, and a picture frame n+1 immediately succeeding the picture frame n in time as indicated by arrow t. FIG. 5 illustrates a case in which a coded pixel (picture element) Pn+1 of the next picture frame n+1 is subjected to interframe encoding based on a target pixel of Pn of the picture frame n.
The processing is based on the premise (an assumption) that there is strong correlation between immediately successive picture frames of a moving-image signal, and basically, pixel data at the same position in adjacent frames have substantially the same value.
If encoding is performed according to simple interframe prediction, only the difference between the pixels Pn and Pn+1, the variance in the image deviation from the above-described premise, is encoded. Hence, it is possible to reduce the amount of data by a method, such as nonlinear quantization or the like, which is similar to one-dimensional DPCM (differential pulse-code modulation).
Such a method compresses the data so that, for example, only transmission of information of “the same as the preceding frame” suffices in the case of a perfectly still picture on the assumption that correlation increases as the values of pixel data of succeeding frames are closer.
However, when correlation between images is low, data corresponding to the predicted error from a predicted correlation value must be transmitted. Hence, when the entire picture frame has much movement, it is possible to temporarily generate a large amount of data.
Accordingly, in the case of a portable image recording apparatus provided with the above-described digital signal processing system, various kinds of image processing using time-base correlation between images cannot be effectively performed due to displacement of picture frames caused by unintended hand movement.